Press hardening steel with high oxidation resistance

ABSTRACT

A steel composition is provided. The steel composition includes 0.1-0.45 wt. % carbon (C), greater than 0-4.5 wt. % manganese (Mn), 0.5-5 wt. % chromium (Cr), 0.5-2.5 wt. % silicon (Si), greater than 0-2 wt. % copper (Cu), and a balance of iron (Fe). The combined concentration of the Mn, Cr, and Cu is greater than about 2 wt. %. The steel composition is configured to form a surface oxide layer comprising oxides of Cr, Si, and Cu after being subjected to press hardening. Press-hardened steel fabricated from the steel composition and a method of fabricating a press-hardened steel component from the steel composition are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority of Chinese Patent Application No. 202010070823.7 filed Jan. 21, 2020. The entire disclosure of the above application is incorporated herein by reference.

INTRODUCTION

This section provides background information related to the present disclosure which is not necessarily prior art.

Press-hardened steel (PHS), also referred to as “hot-stamped steel” or “hot-formed steel,” is one of the strongest steels used for automotive body structural applications, having tensile strength properties of about 1,500 mega-Pascal (MPa). Such steel has desirable properties, including forming steel components with significant increases in strength-to-weight ratios. PHS components have become ever more prevalent in various industries and applications, including general manufacturing, construction equipment, automotive or other transportation industries, home or industrial structures, and the like. For example, when manufacturing vehicles, especially automobiles, continual improvement in fuel efficiency and performance is desirable; therefore, PHS components have been increasingly used. PHS components are often used for forming load-bearing components, like door beams, which usually require high strength materials. Thus, the finished state of these steels are designed to have high strength and enough ductility to resist external forces, such as, for example, resisting intrusion into the passenger compartment without fracturing so as to provide protection to the occupants. Moreover, galvanized PHS components may provide cathodic protection.

Many PHS processes involve austenitization of a sheet steel blank in a furnace, immediately followed by pressing and quenching of the sheet in dies. Austenitization is typically conducted in the range of about 880° C. to 950° C. PHS processes may be indirect or direct. In the direct method, the PHS component is formed and pressed simultaneously between dies, which quenches the steel. In the indirect method, the PHS component is cold-formed to an intermediate partial shape before austenitization and the subsequent pressing and quenching steps. The quenching of the PHS component hardens the component by transforming the microstructure from austenite to martensite. A discontinuous oxide layer often forms on the surface of the component during furnace heating and transferring from the furnace to dies when the component is fabricated from uncoated steel. Therefore, after quenching, the oxide must be removed from the PHS component and the dies. The oxide is typically removed, i.e., descaled, by shot blasting.

The PHS component may be made from bare or coated alloys. Coating the PHS component with, e.g., zinc or Al—Si, provides a protective layer to the underlying steel component. Zinc coatings, for example, offer cathodic protection; the coating acts as a sacrificial layer and corrodes instead of the steel component, even where the steel is exposed. However, zinc-coated PHS generates oxides on PHS component surfaces, which must be removed by shot blasting. Accordingly, alloy compositions that do not require coatings or other treatments are desired.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

The present disclosure relates to press hardening steel with high oxidation resistance.

In various aspects, the current technology provides a steel composition including carbon (C) at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. %; manganese (Mn) at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. %; chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. %; copper (Cu) at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. %; and a balance of iron (Fe), wherein the combined concentration of the Mn, Cr, and Cu is greater than or equal to about 2 wt. %, and wherein the steel composition is configured to form a surface oxide layer including oxides of Cr, Si, and Cu after being subjected to press hardening.

In one aspect, the steel composition further includes nickel (Ni) at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. %, wherein the combined concentration of the Mn, Cr, Cu, and Ni is greater than or equal to about 2 wt. %, and wherein the steel composition is configured to form a surface oxide layer including oxides of Cr, Si, Cu, and Ni after being subjected to press hardening.

In one aspect, the steel composition is free of coatings.

In one aspect, the steel composition further includes an additional element selected from the group consisting of molybdenum (Mo) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; vanadium (V) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; niobium (Nb) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; boron (B) at a concentration of greater than 0 wt. % to less than or equal to about 0.01 wt. %; titanium (Ti) at a concentration of greater than 0 wt. % to less than or equal to about 0.1 wt. %; aluminum (Al) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; and combinations thereof.

In one aspect, the steel composition is in the form a coiled sheet.

In various other aspects, the current technology provides a press-hardened steel including an alloy matrix including carbon (C) at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. %; manganese (Mn) at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. %; chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. %; copper (Cu) at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. %; and a balance of iron (Fe); and an oxide layer formed on a surface of the alloy matrix during hot forming of the press-hardened steel, the oxide layer including oxides of the Cr, Si, and Cu, wherein the oxide layer protects the alloy matrix from oxidation.

In one aspect, the combined concentration of the Mn, Cr, and Cu is greater than or equal to about 2 wt. %.

In one aspect, the matrix further includes nickel (Ni) at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. %, and wherein the oxide layer further includes oxides of the Ni.

In one aspect, the oxide layer is uniform and continuous.

In one aspect, the oxide layer has a thickness of greater than or equal to about 1 nm to less than or equal to about 10 μm.

In one aspect, the matrix has a microstructure including greater than or equal to about 90 vol. % martensite, and a balance including retained austenite and optionally ferrite, wherein when the balance includes ferrite, the ferrite has a concentration of greater than 0 vol. % to less than or equal to about 5 vol. %.

In one aspect, the matrix further includes an additional element selected from the group consisting of molybdenum (Mo) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; vanadium (V) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; niobium (Nb) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; boron (B) at a concentration of greater than 0 wt. % to less than or equal to about 0.01 wt. %; titanium (Ti) at a concentration of greater than 0 wt. % to less than or equal to about 0.1 wt. %; aluminum (Al) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; and combinations thereof.

In one aspect, the press-hardened steel is an automobile part.

In yet various other aspects, the current technology provides a method of fabricating a press-hardened steel component, the method including heating a blank to a temperature of greater than or equal to about 880° C. to less than or equal to about 950° C. to form a heated blank, the blank including a steel composition including carbon (C) at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. %; manganese (Mn) at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. %; chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. %; copper (Cu) at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. %; and a balance of iron (Fe); pressing the heated blank in a die to form a structure having a predetermined shape from the heated blank; and quenching the structure to a temperature less than or equal to about a martensite finish (Mf) temperature of the steel composition and greater than or equal to about room temperature to form the press-hardened steel component, wherein the press-hardened steel component includes an alloy matrix including the C, Mn, Cr, Si, Cu, and Fe, an oxide layer formed on a surface of the alloy matrix, the oxide layer being continuous and uniform, including oxides of the Cr, Si, and Cu, and being configured to resist oxidation, and a microstructure includes greater than or equal to about 90 vol. % martensite, and wherein the press-hardened steel component is formed without descaling and is free of a coating.

In one aspect, the blank and the matrix further include nickel (Ni) at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. %, and wherein the oxide layer further includes oxides of the Ni.

In one aspect, the press-hardened steel component includes about 0.2 wt. % C, about 1.5 wt. % Mn, about 1.5 wt. % Cr, about 1.5 wt. % Si, about 0.8 wt. % Ni, about 0.3 wt. % Cu, and about 0.03 wt. % Nb.

In one aspect, the combined concentration of the Mn, Cr, Cu, and Ni in the blank and in the matrix is greater than or equal to about 2 wt. %.

In one aspect, the microstructure of the press-hardened steel component further includes greater than about 0 vol. % to less than or equal to about 10 vol. % retained austenite, and greater than or equal to about 0 vol. % to less than or equal to about 5 vol. % ferrite.

In one aspect, the method is free of a secondary heat treatment after the quenching.

In one aspect, the press-hardened steel component is an automobile part selected from the group consisting of a wheel, a pillar, a bracket, a bumper, a roof rail, a rocker rail, a rocker, a control arm, a beam, a tunnel, a step, a subframe member, a pan, a panel, and a reinforcement panel.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 is a flow diagram illustrating a method of making a press-hardened steel component according to various aspects of the current technology.

FIG. 2 is a graph showing temperature versus time for a hot pressing method used to process a steel composition according to various aspects of the current technology.

FIG. 3 is an illustration of a press-hardened steel according to various aspects of the current technology.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, elements, compositions, steps, integers, operations, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Although the open-ended term “comprising,” is to be understood as a non-restrictive term used to describe and claim various embodiments set forth herein, in certain aspects, the term may alternatively be understood to instead be a more limiting and restrictive term, such as “consisting of” or “consisting essentially of” Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and/or process steps, the present disclosure also specifically includes embodiments consisting of, or consisting essentially of, such recited compositions, materials, components, elements, features, integers, operations, and/or process steps. In the case of “consisting of,” the alternative embodiment excludes any additional compositions, materials, components, elements, features, integers, operations, and/or process steps, while in the case of “consisting essentially of,” any additional compositions, materials, components, elements, features, integers, operations, and/or process steps that materially affect the basic and novel characteristics are excluded from such an embodiment, but any compositions, materials, components, elements, features, integers, operations, and/or process steps that do not materially affect the basic and novel characteristics can be included in the embodiment.

Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated.

When a component, element, or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other component, element, or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,” “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. For example, “about” may comprise a variation of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in certain aspects, optionally less than or equal to 0.1%.

In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges.

Example embodiments will now be described more fully with reference to the accompanying drawings.

As discussed above, there are certain disadvantages associated with descaling uncoated press-hardened steels and coating press-hardened steels. Accordingly, the current technology provides a steel composition that is configured to be hot stamped into a press-hardened component having a predetermined shape without coatings and without a need to perform descaling.

The steel composition is in the form of a coil or sheet and comprises carbon (C), manganese (Mn), chromium (Cr), silicon (Si), copper (Cu), and iron (Fe). In some aspects, the steel composition also comprises nickel (Ni). During a hot stamping process, portions of the Cr, Si, Cu, and Ni (when present) migrate to a surface of the steel composition and combine with atmospheric oxygen to form a continuous oxide layer comprising an oxide or oxides enriched with the portions of the Cr, Si, Cu, and Ni (when present). The oxides layer resists, i.e., prevents, inhibits, or minimizes, further oxidation. Put another way, the oxide layer protects the press-hardened steel from oxidation. Therefore, descaling steps, such as shot blasting or sand blasting, are not required.

The C is present in the steel composition at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. % or greater than or equal to about 0.1 wt. % to less than or equal to about 0.3 wt. % and subranges thereof. In various aspects, the steel composition comprises C at a concentration of about 0.1 wt. %, about 0.12 wt. %, about 0.14 wt. %, about 0.16 wt. %, about 0.18 wt. %, about 0.2 wt. %, about 0.22 wt. %, about 0.24 wt. %, about 0.26 wt. %, about 0.28 wt. %, about 0.3 wt. %, 0.32 wt. %, about 0.34 wt. %, about 0.36 wt. %, about 0.38 wt. %, about 0.4 wt. %, about 0.42 wt. %, about 0.44 wt. %, or about 0.45 wt. %.

The Mn is present in the steel composition at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. % or greater than or equal to about 1 wt. % to less than or equal to about 2 wt. % and subranges thereof In various aspects, the steel composition comprises Mn at a concentration of about 0.02 wt. %, about 0.04 wt. %, 0.06 wt. %, 0.08 wt. %, about 1 wt. %, about 1.2 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6 wt. %, about 1.8 wt. %, about 2 wt. %, about 2.2 wt. %, about 2.4 wt. %, about 2.5 wt. %, about 2.6 wt. %, about 2.8 wt. %, about 3 wt. %, about 3.2 wt. %, about 3.4 wt. %, about 3.5 wt. %, about 3.6 wt. %, about 3.8 wt. %, about 4 wt. %, about 4.2 wt. %, about 4.4 wt. %, or about 4.5 wt. %.

The Cr is present in the steel composition at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. % greater than or equal to about 1 wt. % to less than or equal to about 3 wt. % and subranges thereof. In various aspects, the steel composition comprises Cr at a concentration of about 0.05 wt. %, 0.06 wt. %, 0.08 wt. %, about 1 wt. %, about 1.2 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6 wt. %, about 1.8 wt. %, about 2 wt. %, about 2.2 wt. %, about 2.4 wt. %, about 2.5 wt. %, about 2.6 wt. %, about 2.8 wt. %, about 3 wt. %, about 3.2 wt. %, about 3.4 wt. %, about 3.5 wt. %, about 3.6 wt. %, about 3.8 wt. %, about 4 wt. %, about 4.2 wt. %, about 4.4 wt. %, about 4.5 wt. %, about 4.6 wt. %, about 4.8 wt. %, or about 5 wt. %.

The Si is present in the steel composition at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % and subranges thereof In various aspects, the steel composition comprises Si at a concentration of about 0.5 wt. %, about 0.6 wt. %, about 0.7 wt. %, about 0.8 wt. %, about 0.9 wt. %, about 1 wt. %, about 1.1 wt. %, about 1.2 wt. %, about 1.3 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6 wt. %, about 1.7 wt. %, about 1.8 wt. %, about 1.9 wt. %, about 2 wt. % about 2.1 wt. %, about 2.2 wt. %, about 2.3 wt. %, about 2.4 wt. %,or about 2.5 wt. %.

The Cu is present in the steel composition at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. % or greater than or equal to about 0.1 wt. % to less than or equal to about 0.5 wt. % and subranges thereof. In various aspects, the steel composition comprises Si at a concentration of about 0.1 wt. %, about 0.2 wt. %, about 0.3 wt. %, about 0.4 wt. %, about 0.5 wt. %, about 0.6 wt. %, about 0.7 wt. %, about 0.8 wt. %, about 0.9 wt. %, about 1 wt. %, about 1.1 wt. %, about 1.2 wt. %, about 1.3 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6 wt. %, about 1.7 wt. %, about 1.8 wt. %, about 1.9 wt. %, or about 2 wt. %.

When included, the Ni is present in the steel composition at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. % or greater than or equal to about 0.5 wt. % to less than or equal to about 1.5 wt. % and subranges thereof. In various aspects, the steel composition comprises Ni at a concentration of about 0.02 wt. %, about 0.04 wt. %, 0.06 wt. %, 0.08 wt. %, about 0.1 wt. %, about 0.12 wt. %, about 0.14 wt. %, about 0.16 wt. %, about 0.18 wt. %, about 1 wt. %, about 1.2 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6 wt. %, about 1.8 wt. %, about 2 wt. %, about 2.2 wt. %, about 2.4 wt. %, about 2.5 wt. %, about 2.6 wt. %, about 2.8 wt. %, about 3 wt. %, about 3.2 wt. %, about 3.4 wt. %, about 3.5 wt. %, about 3.6 wt. %, about 3.8 wt. %, about 4 wt. %, about 4.2 wt. %, about 4.4 wt. %, about 4.6 wt. %, about 4.8 wt. %, or about 5 wt. %.

The Fe makes up the balance of the steel composition.

In some aspects, the combined concentration of the Mn, Cr, and Cu in the steel composition is greater than or equal to about 2 wt. %. When the steel composition comprises the Ni, the combined concentration of the Mn, Cr, Cu, and Ni in the steel composition is greater than or equal to about 2 wt. %. In other aspects, the combined concentration of the Cr, Si, and Cu in the steel composition is greater than or equal to about 2 wt. %. When the steel composition comprises the Ni, the combined concentration of the Cr, Si, Cu, and Ni in the steel composition is greater than or equal to about 2 wt. %.

In various aspects, the steel composition further comprises molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. % and subranges thereof. In various aspects, the steel composition comprises Mo at a concentration of about 0.05 wt. %, about 0.1 wt. %, 0.15 wt. %, 0.2 wt. %, about 0.25 wt. %, about 0.3 wt. %, about 0.35 wt. %, about 0.4 wt. %, about 0.45 wt. %, or about 0.5 wt. %, about 0.55 wt. %, about 0.6 wt. %, about 0.65 wt. %, about 0.7 wt. %, about 0.75 wt. %, about 0.8 wt. %, about 0.85 wt. %, about 0.9 wt. %, about 0.95 wt. %, or about 1 wt. %. In some aspects, the steel composition is substantially free of Mo. As used herein, “substantially free” refers to trace component levels, such as levels of less than or equal to about 0.1 wt. % or levels that are not detectable.

In various aspects, the steel composition further comprises vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. % and subranges thereof In various aspects, the steel composition comprises V at a concentration of about 0.05 wt. %, about 0.1 wt. %, 0.15 wt. %, 0.2 wt. %, about 0.25 wt. %, about 0.3 wt. %, about 0.35 wt. %, about 0.4 wt. %, about 0.45 wt. %, or about 0.5 wt. %, about 0.55 wt. %, about 0.6 wt. %, about 0.65 wt. %, about 0.7 wt. %, about 0.75 wt. %, about 0.8 wt. %, about 0.85 wt. %, about 0.9 wt. %, about 0.95 wt. %, or about 1 wt. %. In some aspects, the steel composition is substantially free of V.

In various aspects, the steel composition further comprises niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. % or greater than or equal to about 0.02 wt. % to less than or equal to about 0.05 wt. % and subranges thereof In various aspects, the steel composition comprises Nb at a concentration of about 0.01 wt. %, about 0.02 wt. %, about 0.04 wt. %, about 0.06 wt. %, about 0.08 wt. %, about 0.1 wt. %, about 0.15 wt. %, about 0.2 wt. %, about 0.25 wt. %, about 0.3 wt. %, about 0.35 wt. %, about 0.4 wt. %, about 0.45 wt. %, or about 0.5 wt. %. In some aspects, the steel composition is substantially free of Nb.

In various aspects, the steel composition further comprises aluminum (Al) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. % and subranges thereof. In various aspects, the steel composition comprises Al at a concentration of about 0.01 wt. %, about 0.02 wt. %, about 0.04 wt. %, about 0.06 wt. %, about 0.08 wt. %, about 0.1 wt. %, about 0.15 wt. %, about 0.2 wt. %, about 0.25 wt. %, about 0.3 wt. %, about 0.35 wt. %, about 0.4 wt. %, about 0.45 wt. %, or about 0.5 wt. %. In some aspects, the steel composition is substantially free of Al.

The steel composition can also include unavoidable impurities. As used herein, “impurities” are elements having a concentration of less than or equal to about 0.1 wt. % that are not intentionally added to the steel composition. Therefore, when not intentionally included in the steel composition and when having a concentration of less than or equal to about 0.1 wt. %, the Mo, V, Nb, and Al are impurities. Non-limiting examples of other impurities include boron (B) and titanium (Ti). For example, the steel composition includes B at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. % and Ti at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.1 wt. %. When the B and Ti are individually not intentionally included in the steel composition, they are impurities.

The steel composition can include various combinations of C, Mn, Cr, Si, Cu, Ni, Mo, V, Nb, Al, B, Ti, and Fe (where the C, Mn, Cr, Si, Cu, and Fe are required components) at their respective concentrations described above. In some aspects, the steel composition consists essentially of C, Mn, Cr, Si, Cu, and Fe or C, Mn, Cr, Si, Cu, Ni, and Fe. As described above, the term “consists essentially of” means the steel composition excludes additional compositions, materials, components, elements, and/or features that materially affect the basic and novel characteristics of the steel composition, such as the steel composition not requiring coatings or descaling when formed into a press-hardened steel component, but any compositions, materials, components, elements, and/or features that do not materially affect the basic and novel characteristics of the steel composition can be included in the aspect, such as impurities as defined above. Therefore, when the steel composition consists essentially of C, Mn, Cr, Si, Cu, and Fe or C, Mn, Cr, Si, Cu, Ni, and Fe, the steel composition can also include any combination of Mo, V, Nb, Al, B, and Ti, as provided above, that does not materially affect the basic and novel characteristics of the steel composition. In other aspects, the steel composition consists of C, Mn, Cr, Si, Cu, and Fe or C, Mn, Cr, Si, Cu, Ni, and Fe at their respective concentrations described above and optionally at least one of Mo, V, Nb, Al, B, and Ti at their respective concentrations described above. Other elements that are not described herein can also be included as impurities, provided that they do not materially affect the basic and novel characteristics of the steel composition.

In one aspect, the steel composition comprises, consists essentially of, or consists of C, Mn, Cr, Si, Cu, and Fe.

In one aspect, the steel composition comprises, consists essentially of, or consists of C, Mn, Cr, Si, Cu, Fe, and at least one of Ni, Mo, V, Nb, Al, B, or Ti.

In one aspect, the steel composition comprises, consists essentially of, or consists of C, Mn, Cr, Si, Cu, Ni, and Fe.

In one aspect, the steel composition comprises, consists essentially of, or consists of C, Mn, Cr, Si, Cu, Ni, Fe, and at least one of Mo, V, Nb, Al, B, or Ti.

In one aspect, the steel composition comprises, consists essentially of, or consists of about 0.2 wt. % C, about 1.5 wt. % Mn, about 1.5 wt. % Cr, about 1.5 wt. % Si, about 0.8 wt. % Ni, about 0.3 wt. % Cu, and about 0.03 wt. % Nb.

With reference to FIG. 1, the current technology also provides a method 10 of fabricating a press-hardened steel component. More particularly, the method includes hot pressing the steel composition described above to form the press-hardened steel component. The steel composition is processed in a bare form, i.e., without any coatings, such as Al—Si or Zn (galvanized) coatings. Moreover, the method does not result in the formation of scale on the press-hardened steel component and is free from a descaling step, i.e., free from shot blasting, sand blasting, or any other method for preparing a smooth and homogenous surface. The press-hardened steel component can be any component that is generally made by hot stamping, such as, a vehicle part, for example. Non-limiting examples of vehicles that have parts suitable to be produced by the current method include bicycles, automobiles, motorcycles, boats, tractors, buses, mobile homes, campers, gliders, airplanes, and tanks. In various aspects, the press-hardened steel component is an automobile part selected from the group consisting of a wheel, a pillar, a bracket, a bumper, a roof rail, a rocker rail, a rocker, a control arm, a beam, a tunnel, a step, a subframe member, a pan, a panel, and a reinforcement panel.

The method 10 comprises obtaining a coil 12 of a steel composition according to the present technology and cutting a blank 14 from the coil 12. Although not shown, the blank 14 can alternatively be cut from a sheet of the steel composition. The steel composition is bare, i.e., uncoated. The method 10 also comprises hot pressing the blank 14. In this regard, the method 10 comprises austenitizing the blank 14 by heating the blank 14 in a furnace 16 to a temperature above its upper critical temperature (Ac3) temperature to fully austenitize the steel composition. The heated blank 14 is transferred to a die or press 18, optionally by a robotic arm (not shown). Here, the method 10 comprises stamping the blank 14 in the die or press 18 to form a structure having a predetermined shape and quenching the structure at a rate to a temperature less than or equal to about a martensite finish (Mf) temperature of the steel composition and greater than or equal to about room temperature or ambient temperature to form the press-hardened steel component. The critical cooling rate, i.e., the minimum cooling rate that provides the required microstructure as described below, is about 15° C/s. Therefore, the quenching comprises decreasing the temperature of the structure at a rate of greater than or equal to about 15° C/s.

The method 10 is free of a descaling step. As such, the method 10 does not include, for example, steps of shot blasting or sand blasting. Inasmuch as the steel composition is bare, the press-hardened steel component is free of and does not include, for example, a discontinuous oxide layer, a layer of zinc (Zn), or an aluminum-silicon (Al—Si) coating. As used herein, a “discontinuous oxide layer” is a non-uniform layer or plurality of oxide layer clusters that should be removed from the surface of press-hardened steel components by, e.g., descaling or shot blasting. The method 10 is also free of a secondary heat treatment after the quenching. As discussed in more detail below, the press-hardened steel component comprises press-hardened steel comprising an alloy matrix (having the components of the steel composition), and a uniform and continuous oxide layer comprising oxides of Cr, Si, Cu, and when present, Ni, formed on the oxide layer. By “continuous,” it is meant that the oxide layer covers all, or substantially all (i.e., greater than or equal to about 90%), of the exposed surfaces of the press-hardened steel component. By “uniform,” it is meant that the thickness of the oxide layer varies by less than or equal to about 20%.

FIG. 2 shows a graph 50 that provides additional details about the hot pressing. The graph 50 has a y-axis 52 representing temperature and an x-axis 54 representing time. A line 56 on the graph 50 represents heating conditions during the hot pressing. Here, the blank is heated to a final temperature 58 that is above an upper critical temperature (Ac3) 60 of the steel composition to fully austenitize the steel composition and form a heated blank. The final temperature 58 is greater than or equal to about 880° C. to less than or equal to about 950° C. The heated blank is then transferred to a press or die. During the transfer, the temperature of the heated blank can decrease by greater than or equal to about 100° C. to less than or equal to about 200° C. Therefore, the temperature of the heated blank decreases to about the Ac3 temperature 60 or lower and the and heated blank is stamped or hot-formed into the structure having the predetermined shape and then cooled at a rate of greater than or equal to about 15° Cs⁻¹, greater than or equal to about 20° Cs⁻¹, greater than or equal to about 25° Cs⁻¹, or greater than or equal to about 30° Cs⁻¹, such as at a rate of about 15° Cs⁻¹, about 18° Cs⁻¹, about 20° Cs⁻¹, about 22° Cs⁻¹, about 24° Cs⁻¹, about 26° Cs⁻¹, about 28° Cs⁻¹, about 30° Cs⁻¹, or faster, until the temperature decreases below a martensite finish (Mf) temperature 66, such that the press-hardened steel component is formed. The press-hardened steel component comprises a matrix comprising the steel composition components and the above-described oxide layer formed on the matrix. The press-hardened steel component, i.e., the matrix, has a microstructure comprising greater than or equal to about 90 vol. % martensite, greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % retained austenite, and greater than or equal to about 0 vol. % to less than or equal to about 5 vol. % ferrite. For example, the microstructure can comprise greater than or equal to about 90 vol. % to less than 100 vol. % martensite and the balance comprise (or consists of) a combination of retained austenite and ferrite, but the ferrite is not present at a concentration of greater than about 5 vol. %. Accordingly, in certain aspects the microstructure comprises or consists of martensite, comprises or consists of martensite and retained austenite, or comprises or consists of martensite, retained austenite, and ferrite.

In some aspects, the hot pressing, i.e., the heating, stamping, and quenching, is performed in an aerobic atmosphere. In other aspects, the hot pressing can be performed in an anaerobic atmosphere, such as by supplying an inert gas into at least one of the oven or the die. The inert gas can be any inert gas known in the art, such as nitrogen or argon, as non-limiting examples.

With reference to FIG. 3, the current technology yet further provides a press-hardened steel 80. The press-hardened steel 80 results from hot pressing the steel composition described above by the method described above. As such, the press-hardened steel structure made by the above method is composed of the press-hardened steel 80.

The press-hardened steel 80 comprises a matrix 82 comprising the steel components, and an oxide layer 84 formed on the matrix, wherein the oxide layer 84 is continuous and uniform. It is understood that FIG. 3 only shows a cross section illustration of a portion of the press-hardened steel 80 and that the oxide layer 84 surrounds or coats all, or substantially all, of the alloy matrix 82. The press-hardened steel 80 has an ultimate tensile strength (UTS) of greater than or equal to about 500 MPa, greater than or equal to about 750 MPa, greater than or equal to about 1,000 MPa, greater than or equal to about 1,250 MPa, greater than or equal to about 1,600 MPa, greater than or equal to about 1,700 MPa, or greater than or equal to about 1,800 MPa. In some aspects, the press-hardened steel 80 has a UTS of greater than or equal to about 1,600 MPa and less than or equal to about 2000 MPa.

The alloy matrix 82 comprises the components and their corresponding concentrations of the steel composition described above, but has the above-described microstructure.

The oxide layer 84 is formed on and disposed directly on the matrix 82 as a continuous and uniform layer during the hot pressing process and comprises an oxide enriched with Cr, Si, Cu, and when Ni is present in the steel composition, Ni, including Cr oxides, Si oxides, Cu oxides, and when Ni is present in the steel composition, Ni oxides.

The oxide layer 84 has a thickness T_(OL) of greater than or equal to about 1 nm to less than or equal to about 10 μm, such as a thickness of about 0.001 μm, about 0.01 μm, about 0.05 μm, about 0.1 μm, about 0.15 μm, about 0.25 μm, about 0.3 μm, about 0.35 μm, about 0.4 μm, about 0.45 μm, about 0.5 μm, about 0.55 μm, about 0.6 μm, about 0.65 μm, about 0.7 μm, about 0.75 μm, about 0.8 μm, about 0.85 μm, about 0.9 μm, about 0.95 μm, about 1 μm, about 1.5 μm, about 2 μm, about 2.5 μm, about 3 μm, about 3.5 μm, about 4 μm, about 4.5 μm, about 5 μm, about 5.5 μm, about 6 μm, about 6.5 μm, about 7 μm, about 7.5 μm, about 8 μm, about 8.5 μm, about 9 μm, about 9.5 μm, or about 10 μm.

In certain variations, the oxide layer 84 is continuous, uniform, and homogenous. Therefore, the oxide layer 84 provides an exposed surface that is free of or substantially free of (i.e., comprising less than or equal to about 10% of the exposed surface) discontinuous oxide layers, and there is no need for it to be descaled by, for example, shot blasting or sand blasting. Moreover, the oxide layer 84 resists (i.e., prevent, inhibits, or minimizes) further surface oxidation.

The press-hardened steel 80 does not include or is free of any layer that is not derived from the steel composition or the matrix 82, as discussed above.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

What is claimed is:
 1. A steel composition comprising: carbon (C) at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. %; manganese (Mn) at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. %; chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. %; copper (Cu) at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. %; and a balance of iron (Fe), wherein the combined concentration of the Mn, Cr, and Cu is greater than or equal to about 2 wt. %, and wherein the steel composition is configured to form a surface oxide layer comprising oxides of Cr, Si, and Cu after being subjected to press hardening.
 2. The steel composition according to claim 1, further comprising: nickel (Ni) at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. %, wherein the combined concentration of the Mn, Cr, Cu, and Ni is greater than or equal to about 2 wt. %, and wherein the steel composition is configured to form a surface oxide layer comprising oxides of Cr, Si, Cu, and Ni after being subjected to press hardening.
 3. The steel composition according to claim 2, wherein the steel composition is free of coatings.
 4. The steel composition according to claim 1, further comprising an additional element selected from the group consisting of: molybdenum (Mo) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; vanadium (V) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; niobium (Nb) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; boron (B) at a concentration of greater than 0 wt. % to less than or equal to about 0.01 wt. %; titanium (Ti) at a concentration of greater than 0 wt. % to less than or equal to about 0.1 wt. %; aluminum (Al) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; and combinations thereof.
 5. The steel composition according to claim 1, wherein the steel composition is in the form a coiled sheet.
 6. A press-hardened steel comprising: an alloy matrix comprising: carbon (C) at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. %; manganese (Mn) at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. %; chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. %; copper (Cu) at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. %; and a balance of iron (Fe); and an oxide layer formed on a surface of the alloy matrix during hot forming of the press-hardened steel, the oxide layer comprising oxides of the Cr, Si, and Cu, wherein the oxide layer protects the alloy matrix from oxidation.
 7. The press-hardened steel according to claim 6, wherein the combined concentration of the Mn, Cr, and Cu is greater than or equal to about 2 wt. %.
 8. The press-hardened steel according to claim 6, wherein the matrix further comprises: nickel (Ni) at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. %, and wherein the oxide layer further comprises oxides of the Ni.
 9. The press-hardened steel according to claim 8, wherein the oxide layer is uniform and continuous.
 10. The press-hardened steel according to claim 6, wherein the oxide layer has a thickness of greater than or equal to about 1 nm to less than or equal to about 10 μm.
 11. The press-hardened steel according to claim 6, wherein the matrix has a microstructure comprising greater than or equal to about 90 vol. % martensite, and a balance comprising retained austenite and optionally ferrite, wherein when the balance comprises ferrite, the ferrite has a concentration of greater than 0 vol. % to less than or equal to about 5 vol. %.
 12. The press-hardened steel according to claim 6, wherein the matrix further comprises an additional element selected from the group consisting of: molybdenum (Mo) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; vanadium (V) at a concentration of greater than 0 wt. % to less than or equal to about 1 wt. %; niobium (Nb) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; boron (B) at a concentration of greater than 0 wt. % to less than or equal to about 0.01 wt. %; titanium (Ti) at a concentration of greater than 0 wt. % to less than or equal to about 0.1 wt. %; aluminum (Al) at a concentration of greater than 0 wt. % to less than or equal to about 0.5 wt. %; and combinations thereof.
 13. An automobile part comprising the press-hardened steel according to claim
 6. 14. A method of fabricating a press-hardened steel component, the method comprising: heating a blank to a temperature of greater than or equal to about 880° C. to less than or equal to about 950° C. to form a heated blank, the blank comprising a steel composition comprising: carbon (C) at a concentration of greater than or equal to about 0.1 wt. % to less than or equal to about 0.45 wt. %; manganese (Mn) at a concentration of greater than 0 wt. % to less than or equal to about 4.5 wt. %; chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 5 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. %; copper (Cu) at a concentration of greater than 0 wt. % to less than or equal to about 2 wt. %; and a balance of iron (Fe), pressing the heated blank in a die to form a structure having a predetermined shape from the heated blank; and quenching the structure to a temperature less than or equal to about a martensite finish (M_(f)) temperature of the steel composition and greater than or equal to about room temperature to form the press-hardened steel component, wherein the press-hardened steel component comprises: an alloy matrix comprising the C, Mn, Cr, Si, Cu, and Fe, an oxide layer formed on a surface of the alloy matrix, the oxide layer being continuous and uniform, comprising oxides of the Cr, Si, and Cu, and being configured to resist oxidation, and a microstructure comprises greater than or equal to about 90 vol. % martensite, and wherein the press-hardened steel component is formed without descaling and is free of a coating.
 15. The method according to claim 14, wherein the blank and the matrix further comprise: nickel (Ni) at a concentration of greater than 0 wt. % to less than or equal to about 5 wt. %, and wherein the oxide layer further comprises oxides of the Ni.
 16. The method according to claim 15, wherein the press-hardened steel component comprises about 0.2 wt. % C, about 1.5 wt. % Mn, about 1.5 wt. % Cr, about 1.5 wt. % Si, about 0.8 wt. % Ni, about 0.3 wt. % Cu, and about 0.03 wt. % Nb.
 17. The method according to claim 15, wherein the combined concentration of the Mn, Cr, Cu, and Ni in the blank and in the matrix is greater than or equal to about 2 wt. %.
 18. The method according to claim 14, wherein the microstructure of the press-hardened steel component further comprises greater than about 0 vol. % to less than or equal to about 10 vol. % retained austenite, and greater than or equal to about 0 vol. % to less than or equal to about 5 vol. % ferrite.
 19. The method according to claim 14, wherein the method is free of a secondary heat treatment after the quenching.
 20. The method according to claim 14, wherein the press-hardened steel component is an automobile part selected from the group consisting of a wheel, a pillar, a bracket, a bumper, a roof rail, a rocker rail, a rocker, a control arm, a beam, a tunnel, a step, a subframe member, a pan, a panel, and a reinforcement panel. 